The present invention relates generally to printing presses and more particularly to the use of the electrorheological effect in printing presses.
In some printing presses the ink may be metered from an ink fountain in a long ink train consisting of as many as 8 to 22 adjacent rolls. The task of the long train ink is to reduce the thickness of the ink layer to the appropriate level corresponding with a desired ink density on the printed substrate.
In other printing presses the ink is metered instead using a short inking unit, or an Anilox metering device, removing the need for the long ink train. In a short inking unit, an engraved metering roll (i.e. Anilox roll) is used in order to meter ink from an ink reservoir. The ink fills engraved cells of a ceramic layer on the circumference of the metering roller to form an ink layer having a predetermined thickness. Typically, in waterless printing, for example, the ink is then transferred to a form roll before being deposited onto a silicon printing plate affixed to a plate cylinder. In some cases, the ink layer is transferred directly to the printing plate from the metering roll.
A disadvantage of using either an Anilox metering roll or a long ink train is that the ink density (i.e. thickness of the ink layer on the printed web surface) cannot easily be adjusted. To adjust the thickness of the ink layer in a short inking unit, one can replace the metering roll with another metering roll having the desired engraving depth. Another possibility is to change the slip between the metering roll and the form roll or rolls, for example, by rotating the metering roll at a faster or slower speed relative to the form roll. A third known possible way to change the ink density is to change the temperature of the metering roll and the form roll, or for a long ink train, to change the temperature several or all of the adjacent inking rolls. Changing the temperature of the rolls changes the temperature of the ink and thus its viscosity. The ink viscosity determines how much ink will be transferred from the metering roll to the form roll.
Each of these methods for adjusting the ink density is time consuming, costly, and may lead to undesirable results. For example, it takes a long time to change the temperature of the rolls and the temperature-viscosity relationship can be different depending the ink types, and the different ink colors (cyan, magenta, yellow, and black). Replacing a metering roll with another metering roll of a different thickness is expensive and requires that the press is non-operational for a relatively long period of time. Finally, changing the slip between rollers requires experimentation to determine the proper slippage for each ink that can vary depending on additional press conditions, and can lead to inaccurate results.
Waterless inks are typically preferred for sheetfed lithographic processes. One reason is that waterless inks have a higher viscosity and tack level that allows for 100% release from the mostly silicone non-printing areas of the plates. After transferring the ink from the image area of the printing plate to the blanket, the ink is printed on the paper. Because of the high viscosity and tack of the ink, the ink can generate a so-called picking problem, in which the ink pulls out parts of the surface of the paper as the surface of the blanket separates from the paper. The small bits of paper accumulate on the surface of the blanket and plate cylinders, decreasing printing quality and requiring frequent cleaning of the blanket and plate cylinders. The picking problem is especially severe when lower quality paper is used, for example, newspaper. In these printing applications in which lower quality paper is used, the fibers of paper picked by the ink appear as lint, (the phenomenon is often referred to as linting), making efficient printing nearly impossible. Reducing the viscosity of the ink has the positive effect of reducing the Tinting in these processes, but also the negative effect of the ink toning. That is, as ink is transferred from the form roll to the plate cylinder, the less viscous ink adheres to the non-image areas of a platexe2x80x94where it is not supposed to adherexe2x80x94and is thus transferred to areas of the paper where it should not print.
The electrorheological effect is a known phenomenon by which certain fluids change their viscosity and flow characteristics when an electrical field is applied. Certain fluids having electrorheological properties exhibit a positive electrorheological effect when subjected to an electric field in that their viscosity is increased. Other fluids having electrorheological properties exhibit a negative electrorheological effect when subjected to an electric field in that their viscosity decreases. German Patent Document No. DE 44 16 822 describes a method of controlling and drying of coating materials in machines and controlling the inking in printing machines. The document describes generally utilizing the electrorheological effect of electrical fields, but does not describe how the electrical field should be applied to rolls of an ink train. Printing presses including a short inking unit typically use a metering roll having a non-conducting ceramic outer surface and a form roll having a non-conducting rubber outer surface. Applying an electric field across a nip between surfaces of such rollers would require very high voltages and therefore is not practical.
The present invention provides a method of controlling a transfer of an ink having electrorheological properties between rolls in a printing press. The method includes electrically connecting a first electrode to a first roll, electrically connecting a second electrode to a second roll, circumscribing each of the first and second rolls with an electrically conductive layer, establishing a rotational movement between the first and second rolls so as to enable a transfer of the ink from the first roll to the second roll across a first nip defined between the first and second rolls, and applying a voltage across the first and second electrodes so as to create a first electric field at the first nip for influencing a flow of the ink from the first roll to the second roll. The present invention, of course, extends to printing presses including more rolls beyond the first and second rolls specifically referred to here.
The first roll may be a metering roll and the second roll may be a form roll. The first roll may be a form roll and the second roll may be a plate cylinder. Alternatively, the first roll may be a blanket roll and the second roll may be a second blanket cylinder or an impression cylinder. The circumscribing of each of the first and second rolls may include circumscribing the metering roll with an electrically conductive ceramic layer and/or circumscribing the form roll with an electrically conductive rubber layer.
The method may also include electrically connecting a third electrode to a third roll, circumscribing the third roll with an electrically conductive layer, establishing a rotational movement between the second roll and the third roll so as to enable a transfer of the ink across a second nip between the second and third rolls, and applying a second electric field at the second nip to influence a flow of the ink from the second roll to the third roll.
In the case of a printing press having a third roll, the first roll may be a metering roll, the second roll may be a form roll, and the third roll may be a plate cylinder. The electric field may influence the flow of the ink by changing a viscosity of the ink, such as by increasing or decreasing a viscosity of the ink.
The present invention also provides a printing unit that includes a first roll and a second roll configured to enable a transfer of an ink having rheological properties from the first roll to the second roll across a first nip defined between the first and second rolls, the first and second rolls each having an electrically conductive layer circumscribed around them. The printing unit also includes a first electrode electrically connected to the first roll, a second electrode electrically connected to the second roll, and a first voltage source electrically connected to the first and second electrodes to apply an electrical field at the first nip.
The first roll in the printing unit may be a metering roll and the second roll may be a form roll. The first roll may also be a form roll and the second roll may be a print cylinder. Alternatively, the first roll may be a blanket cylinder and the second roll may be a second blanket cylinder or an impression cylinder.
The printing unit may also include a third roll configured to enable a transfer of the ink from the second roll to the third roll across a second nip defined between the first and second rolls, and a second voltage source configured to apply an electric field at the second nip. In the case of the printing unit having a third roll, the first roll may be a metering roll, the second roll may be a form roll and the third roll may be a plate cylinder.
The print unit may also include an ink reservoir containing the ink, wherein the metering roll is configured to meter the ink from the ink reservoir. The second voltage source may be electrically connected to the second and third rolls. The electrically conductive layer of the first roll may include an electrically conductive ceramic layer, and the electrically conductive layer of the second roll may include an electrically conductive rubber layer.